It is known to form a platinum-rhodium alloy from an aqueous bath containing platinum and rhodium. Illustrative of this prior art is U.S. Pat. No. 2,027,358 to Powell et al., U.S. Pat. No. 3,276,976 to Juliard, U.S. Pat. No. 3,671,408 to Martini and U.S. Pat. No. 3,748,712 to Karawin. Also known is the formation of a molten cyanide bath containing a platinum group metal, with U.S. Pat. No. 2,093,406 to Atkinson and U.S. Pat. No. 4,149,942 to von Stutterheim being exemplary of this type of art. Also, we have given oral presentations of the methods described below to prepare the individual platinum- and rhodium-containing molten cyanide baths in 1976 and 1978.
The deposition of coatings of various platinum metals from molten alkali cyanide baths is reported in R. N. Rhoda, Plating, Vol. 49, pp. 69-71 (January, 1962), and U.S. Pat. No. 3,309,292 to R. L. Andrews et al. Additionally, the prior art, as illustrated by U.S. Pat. No. 3,547,789 to R. L. Andrews, G. R. Smith, C. B. Kenahan and D. Schlain, discloses obtaining an alloy of platinum or palladium with another platinum group metal by electrolytically dissolving the platinum or palladium, and the other platinum group metal, separately in a molten alkali metal cyanide bath to form a metal cyanide complex, combining the resulting baths, and electroplating the alloy onto the desired substrate. In this Andrews patent, it is essential in the preparation of the metal cyanide complex to expose the molten bath to air or oxygen either before or during the introduction of primary metal, and it is essential to expose the bath to air or oxygen at temperatures of 600.degree. C. or less to ensure the continued and repeated formation of coatings of 5 or more mils in thickness. Only example 5 shows operation of a plating bath in the absence of air, and this bath is a palladium bath.
In the later work of two of the inventors of this Andrews patent, it was reported that the preparation and use of a molten platinum cyanide bath in an inert atmosphere, using either sodium cyanide or a sodium and potassium cyanide mixture, was unsuccessful, since the deposits obtained were always thin, and the cathode current efficiencies were only 1 or 2%. It was also reported that in the absence of air, almost all of the platinum removed from the anodes precipitated and settled to the bottom of the melt. Similarly, it was reported that attempts to prepare and use molten palladium cyanide baths in inert atmospheres were unsuccessful. These reports are contained in D. Schlain, F. X. McCawley, and G. R. Smith, Report of Investigations 8249, Bureau of Mines Report of Investigations (1977).
Also reported in this publication is prior work showing that an inert atmosphere is essential for the formation of a satisfactory coating of iridium, as well as rhodium, from a molten cyanide bath. This prior work is G. R. Smith, et al., Plating, Vol. 56, pp. 805-808 (July 1969). Finally, it is known in the prior art to construct a galvanic cell using sodium beta-alumina as a selective ion transport membrane. Illustrative of this type of prior art is D. R. Flinn, et al., Journal of the Electrochemical Society, Vol. 123(7), pp. 978-981 (July 1976).
However, this prior art does not provide a platinum-rhodium alloy of predetermined composition and thickness. This prior art does not provide a platinum-rhodium alloy coating that is substantially uniform throughout the coating. For example, the Andrews patent discussed above provides in example 11 a platinum-ruthenium alloy which varied in ruthenium concentration from 2 to 7%.